Video showing the final secret in the Paradox mission while it's the Daily Heroic in The Taken King/TTK expansion for Destiny. After you obtain the previous 3 Memories/Dead Ghost, this path will open up after the third and allow you to enter the portal where you need to defeat the Blighted Descendant. After you do this, you'll have access to a chest which you can turn in at the Tower for the Mystery: Praedyth's Door grimiore.
All 3 other ghosts can be found in the Destiny playlist on my channel. ^^ This also leads to the No Time To Explain exotic pulse rifle / NTTE. Full guide for No Time To Explain here: https://www.youtube.com/watch?v=Va5KRSoV9Lw